Current switching sensor detector

ABSTRACT

A sensor for a switching circuit detects the logical state of the switching circuit by monitoring the current flow through the switching circuit. The current flow is conditioned by one or more current limiters and a voltage regulator, coupled in series with the switching circuit. The sensor also includes a current limit control circuit coupled to each of the current limiters. The sensor is effectively shielded from the effect of parasitic capacitance in the switching device because the current flow through the switching circuit reacts immediately and without regard to the level of parasitic capacitance whenever the switching circuit makes a state change.

This application is a continuation of application Ser. No. 10/135,563,filed May 1, 2002, which issued as U.S. Pat. No. 6,775,165 on Aug. 10,2004, the 60/288,038, filed May 1, 2001, the subject matter of which issubject matter of which is hereby incorporated by reference.

FIELD OF INVENTION

The present invention relates to sensing a logic state using a sensor ordetector. More specifically, the present invention relates to a currentsensing architecture for detecting a logic state.

BACKGROUND OF THE INVENTION

One way to detect the logic state of a switching device is to couple thedevice between a power source and ground and measuring the resultingvoltage. For example, in FIG. 1A, power is applied at terminal 101,which is coupled in series with a resistor 102 and a switching device104 to a ground 105. The switching device 104 may be a single switchingdevice, such as a transistor, or a more complex device, such as a seriesof switching devices which form a logic circuit having a logic output.The logic state of the switching device 104 may be determined bymeasuring the voltage at terminal 103. If the voltage at terminal 103 isrelatively high, then the switching device 104 is in a open state.Similarly, if the voltage at terminal 103 is relatively low, then theswitching device 104 is in a closed state. The change in voltage atterminal 103 is related to the current flow rate through the switchingdevice. Thus, the voltage sensing at terminal 103 should be performedonly after the sufficient time has elapsed for the voltage to becomestable after a state change in the switching device 104.

An issue which arises when using a circuit such as illustrated in FIG.1A in a semiconductor device is that of parasitic capacitance. Parasiticcapacitance is a unwanted capacitance resulting from the fabrication ofthe semiconductor device and is typically associated with conductivelines. FIG. 1B illustrates a circuit equivalent to that illustrated inFIG. 1A, but with the parasitic capacitance illustrated explicitlyillustrated as capacitor 106 coupled in parallel to the switching device104 in-between resistor 102 and ground 105. The effect of parasiticcapacitance is to reduce the rate a voltage at node 103 changes overtime as the switching device 104 switches states. For example, if theswitching device 104 were open and then switched to a close position,the voltage a node 103 in FIG. 1B would fall towards its new value at aslower rate than if the parasitic capacitance 106 were not present.Parasitic capacitance, therefore, increases the time required to detecta changed state of the switching device 104.

One method for compensating the reduced switching speed imposed byparasitic capacitance is to provide increased current flow through thecircuit. Increasing the maximum current flow through the switchingdevice 104 discharges the charge stored by the parasitic capacitancefaster when switch 104 is closed and changes capacitor 106 faster whenswitch 104 is opened. Thus, increasing the maximum current flowthroughout the circuit permits the voltage at node 103 to reach a stablestate faster after the switching device 104 has changed its logicalstate. Unfortunately, increasing the maximum current flow also increasesthe power consumption of the circuit. Accordingly, there is a need anddesire for a method and apparatus to quickly and efficiently detect alogic state of a device in an environment having significant parasiticcapacitance.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and method for quicklyand efficiently detecting a logic state of a switching device. Thepresent invention incorporates a series circuit coupling a power supplysource to ground through a current sensing amplifier, at least onecurrent limiter, a voltage regulator, and the switching device. Acurrent limiter control circuit is coupled to the at least one currentlimiter. In an alternate embodiment, two current limiters are used inthe series circuit. The current sensing amplifier measures the currentflowing through the switching device and does not need to wait forcharge stored by the parasitic capacitance to charge or discharge beforesensing a logic level change. Thus, the present invention is not slowedby parasitic capacitance and does not require increased current flow tocompensate for the parasitic capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of exemplaryembodiments of the invention given below with reference to theaccompanying drawings in which:

FIG. 1A is a circuit diagram of a conventional voltage detecting circuitfor a switching device;

FIG. 1B is a circuit diagram of a conventional voltage detecting circuitswitching device in an environment having parasitic capacitance;

FIG. 2 is a block diagram of one embodiment of the present invention;

FIG. 3 is a illustration of the current sensing circuit;

FIG. 4 is a illustration of a current limiter control circuit;

FIG. 5 is an illustration of an alternate embodiment of the currentsensing circuit; and

FIG. 6 is a block diagram of a CAM memory array having CAM cells whichincorporate the match detection circuitry of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Now referring to the drawings, where like reference numerals designatelike elements, there is shown in FIG. 2 a block diagram of the presentinvention 200 coupled to a switching device 104. The present inventionincludes several components coupled in series between a power source(Vdd) and the switching device 104, and between the switching device 104and a ground potential. More specifically, a current sensing amplifier201, a first current limiter 203, and a voltage regulator 205 arecoupled in series between the power source and the switching device 104.Additionally, a second current limiter 204 is coupled between theswitching device 104 and the ground potential. In an alternateembodiment, the second current limiter 204 is not used. The presentinvention also includes a current limit control 202, which is coupled tothe first and second current limiters 203, 204. The switching device 104is shown as a switch, e.g., a transistor switch, however, the switchingdevice 104 may be other devices or circuits which act as a logic levelswitch.

The present invention 200 operates by detecting changes in the current(Is) at current sensing amplifier 201 flowing into a first currentlimiter 203, the output of which is applied to the voltage regulator 205which supplies a voltage regulated current to the switch device 104. Theswitch device 104 is also optionally connected to the second currentlimiter 204 to ground. As will be explained in greater detail below, thefirst and second current limiters 203, 205 cooperate with the currentlimit control circuit 202 to maintain the voltage (Vs) at the upper nodeof the switching device 104 at a predetermined value.

In a steady state with the switching device 104 in an open state, nocurrent flows through the switching device 104 or the voltage regulator205. As the switching device 104 transitions to a closed state,discharge current begins to flow through the switching device 104. Aportion of this discharge current is caused by the charge stored in theparasitic capacitance (FIG. 2, element 106). At the same time, thevoltage regulator 205 attempts to maintain the voltage Vs by supplying acharging current. If the discharge current through the switching device104 is not limited, the magnitude of the discharge current would beequal to the voltage Vs maintained by the voltage regulator 205 dividedby the impedance of the switching device 104 in its closed state.

The FIG. 2 embodiment of the invention includes two current limiters203, 204 controlled by the current limit control 205 circuit to limitcurrent flow and thereby control power usage. In particular, the firstcurrent limiter 203 is used to limit the charging current supplied bythe voltage regulator 205, while the second current limiter 204 is usedto limit the discharge current through the switching device 204. In thepreferred embodiment, the two current limiters 203, 204 are controlledby the current limit control 202 circuit to allow equal amounts ofcharge and discharge currents to flow, thereby permitting the voltage Vsto be set at a predetermined level. The predetermined level is ideally alow level, in order to minimize the amount of charge stored by theparasitic capacitance.

FIGS. 3 and 4 are circuit diagrams illustrating the FIG. 2 exemplaryembodiment of the present invention. More specifically, FIG. 3 shows animplementation of the current sensing amplifier 201, the first andsecond current limiters 203, 204, and the voltage regulator 205, whileFIG. 4 illustrates the current limit control circuit 202.

As shown in FIG. 3, the current sensing amplifier 201 can be constructedas a circuit having five transistors. More specifically, transistors301, 302 form a current mirror whereby current flows through transistors302 mirrors the current flow through transistor 301, while transistors303, 305 form an invertor/comparator which converts the voltage on thedrain of transistor 308 to the logic signal DATA. The transistor 308,which also has its source coupled to a ground potential and its gatecoupled to a DISCHARGE pin, is used to discharge any charge storedwithin the current sensing amplifier 201 due to its own parasiticcapacitance.

The first and second current limiters 203, 204 are implemented as asingle transistor acting as a variable resistor. The first currentlimiter includes transistor 304 which has its gate voltage controlled bythe signal CLREF1, while the second current limiter 204 includestransistor 307, which has its gate voltage controlled by the signalCLREF2. The CLREF1 and CLREF2 signals are governed by the current limitcontrol 202 circuit, explained below with reference to FIG. 4.

The voltage regulator 205 is also implemented using a single transistor306. The transistor 306 has its drain coupled to the drain of transistor304. The transistor 306 has its gate voltage coupled to a DC voltagereference signal VREF. The output impedance of the transistor 306, atthe SENSE pin, is low.

Now referring to FIG. 4, the current limit control circuit 202 may beformed from opamps 401, 402, resistors 403, 404, and transistors309-314. A fixed, temperature stable reference voltage is supplied atthe VREF pin. This reference voltage is applied to both opamps 401, 402.When the circuit 202 is settled, the voltage levels on the inverting(Ain\) pin of each opamp 401, 402 must be equal to the non-inverting(Ain) pin. Under these conditions the voltage across resistor 403 equalsthe voltage across resistor 404. In the preferred embodiment theresistance of both resistors 403, 404 are equal, causing the currentwhich flows through both resistors 403, 404 to be equal as well. Thecurrent which flows through resistor 403 is generated by a currentmirror formed by transistors 309, 311. The current through transistor311 and 309 are equal and controlled by transistor 313 and opamp 401.The current through transistor 313 equals the current through resistor403. The voltage which controls transistor 313 is coupled to CLREF1.

Similarly, the current which flows through resistor 404 is generated bya current mirror formed by transistors 312, 310. The current throughtransistor 311 and 309 are equal and controlled by transistor 314 andopamp 402. The current through transistor 314 equals the current throughresistor 404. The voltage which controls transistor 314 is coupled toCLREF2. In the preferred embodiment the CLREF1 and CLREF2 signals areset so that the current limit in the first and second current limiters203, 204, i.e., the current through transistors 304, 307 are equal, andthere is net no current which would charge or discharge the parasiticcapacitance. A possible modification to the current limiter controlcircuit 202, for use in connection with an alternate embodimentutilizing a single current limiter, is described below in connectionwith FIG. 5.

Referring again to FIG. 3, the switching device 104 is coupled betweenthe SENSE and LIMIT pins. The parasitic capacitance can be thought of asa capacitor coupled between the SENSE pin and ground. When the switchingdevice is in a closed state, current will flow from the SENSE pinthrough the switching device 104 to the LIMIT pin.

The switching device 104 is coupled between the SENSE pin and the LIMITpin (in the embodiment using both current limiters 203, 204) or betweenthe SENSE pin and ground (in the embodiment using only current limiter203). Under either embodiment, the parasitic capacitance of theswitching device 104 can be thought of as a capacitor coupled betweenthe SENSE pin and the ground. When the switching device 104 is in aclosed state, current will flow from the SENSE pin through the switchingdevice 104 to the limit pin. If the current limiters 203, 204 are set tothe same current limit, the parasitic capacitance of the switchingdevice 104 will not be charging or discharging. Thus, the voltage at thesense pin will remain constant.

At the current sensing amplifier 201, the DISCHARGE pin is normally keptat a low logic level. The transistor 308 is therefore behaves like anopen circuit, and permits the small current generated by transistor 302to rapidly charge the parasitic capacitance associated with the currentsensing amplifier 201 (i.e, transistors 301, 302, 303, 305, 308).

When the switching device 104 moves from a closed state to an openstate, no current can flow through the voltage regulator 305 (i.e.,transistor 306). Additionally, since the parasitic capacitanceassociated with both the switching device 104 and the current sensingamplifier 201 are charged, no current flows due to the parasiticcapacitance. Thus, the output produced by the current sensing amplifier201 at the DATA pin is stable and corresponds to the switching device104 being in a open state.

After one (and before the next) current sensing operation, the parasiticcapacitance of the current sensing amplifier 201 must be discharged.This may be done by temporarily placing a high level signal on theDISCHARGE pin, which causes transistor 308 to behave like an closedcircuit, permitting the charge stored in the parasitic capacitance toflow to ground through transistor 308. Since the parasitic capacitanceof the current sensing amplifier 201 is low relative to the parasiticcapacitance of the switching device 104, the parasitic capacitance ofthe current sensing amplifier 201 may be charged or discharged quickly.The state of the DISCHARGE pin is normally toggled high for a briefperiod of time as the switching device 104 changes state. The output atthe DATA pin of the current sensing amplifier 201 is stable a short timeafter the state of the DISCHARGE pin returns low after being toggledhigh as the switching device 104 changes states.

When the switching device 104 moves to a closed state from an openstate, a current begins to immediately flow through the switching device104. A portion of this current flow is from the voltage regulator 205,as the voltage regulator attempts to maintain the voltage at the SENSEpin at a predetermined value. Another portion of the current flow is adischarge current from the parasitic capacitance. The portion of thecurrent which flows through the voltage regulator 205 also flows throughthe first current limiter 203 and the current sensing amplifier 201. Thecurrent flow through transistor 301 is mirrored in transistor 302 and isquickly output as a signal on the DATA pin by inverter 303, 305.

FIG. 5 illustrates an alternate embodiment which does not utilize thesecond current limiter 204. This alternate embodiment features the samecircuitry for the current sensing amplifier 201, the first currentlimiter 203, and the voltage regulator 205. However, since the secondcurrent limiter 204 has been removed, the switching device 104 iscoupled between the SENSE pin and a source of ground potential. In thisembodiment, the CLREF2 signal is not used since there is only a singlecurrent limiter 203, which is controlled by the CLREF1 signal. Althoughthe current limiter control circuit 202 illustrated in FIG. 4 may alsobe used in this embodiment, since it generates control signals CLREF1,CLREF2, in the interest of efficiency the circuit of FIG. 4 may bemodified by eliminating opamp 402, resistor 404, transistors 310, 312,and 314, and node clREF2. The resulting circuit would then only generatethe CLREF1 control signal, which is all that is needed in the singlecurrent limiter embodiment.

The present invention may be used in any application where parasiticcapacitance may be a concern. For example, one such application may bein content addressable memory systems. Referring now to FIG. 6, aportion of a CAM memory array 600 using the present invention isillustrated. The CAM array 600 includes a plurality of CAM cells 601which are arranged in rows 602 and columns 603. Each CAM cell 601includes a match detection circuit 200 which may employ the logic statedetector of the present invention. CAM cells 601 are coupled to eachcolumn 602 via complementary DATA 610 and DATA* 611 lines. Similarly,the CAM cells 601 are coupled to each row via a word line 620 and amatch line 630.

In a CAM, each stored data word may be searched against a target datapattern. For example, the search data may be placed upon the DATA 610and DATA* 611 lines. A search is conducted simultaneously on all datawords in the CAM. The match detection circuit 200 of the presentinvention may be used to detect the match between data on the datalines, and stored data. If the stored and search data do not match, thematch line (which is pre-charged before the search data is asserted onthe DATA 610 and DATA* 611 lines) is discharged through the cell 601.Thus, the match line 620 remains high only when the entire word matchesthe search data.

While the invention has been described in detail in connection with theexemplary embodiment, it should be understood that the invention is notlimited to the above disclosed embodiment. Rather, the invention can bemodified to incorporate any number of variations, alternations,substitutions, or equivalent arrangements not heretofore described, butwhich are commensurate with the spirit and scope of the invention.Accordingly, the invention is not limited by the foregoing descriptionor drawings, but is only limited by the scope of the appended claims.

1. A circuit for detecting a state of a controllable element,comprising: a current sensing amplifier for producing a first current; afirst current limiter, coupled to said current sensing amplifier, forlimiting a current level of a current flowing through said first currentlimiter to be no greater than a first parameter; and a voltageregulator, coupled to said first current limiter, for controlling avoltage at an output node of said voltage regulator to be at a secondparameter; wherein said first current flows in series through said firstcurrent limiter, said voltage regulator, and said controllable element,said current sensing amplifier senses a current level of said firstcurrent and produces an output signal as a function of said currentlevel, said output signal also being indicative of a state of saidcontrollable element.
 2. The circuit of claim 1, wherein saidcontrollable element is a switching circuit.
 3. The circuit of claim 1,further comprising: a second current limiter, coupled to saidcontrollable element, for limiting a current level of a current flowingthrough said second current limiter to be no greater than a thirdparameter; wherein said first current also flows in series through saidsecond current limiter.
 4. The circuit of claim 3, wherein said firstcurrent flows from said first current limiter to said controllableelement, and from said controllable element to said second currentlimiter.
 5. The circuit of claim 1, further comprising: a controlcircuit, coupled to said current sensing amplifier, for controlling alevel of said first parameter.
 6. The circuit of claim 5, wherein saidcontrol circuit is also coupled to said voltage regulator and controls alevel of said second parameter.
 7. The circuit of claim 5, furthercomprising: a second current limiter, coupled to said controllableelement, for limiting a current level of a current flowing through saidsecond current limiter to be no greater than a third parameter; whereinsaid first current also flows in series through said second currentlimiter and said control circuit also controls a level of said thirdparameter.
 8. The circuit of claim 7, wherein said control circuit isalso coupled to said voltage regulator and controls a level of saidsecond parameter.
 9. The circuit of claim 8, wherein said controlcircuit maintains a same level for said first and third parameters. 10.The circuit of claim 1, wherein said current sensing amplifiercomprises: an input node for receiving an input power; a current mirrorhaving an input coupled to said input node, a first output forgenerating said first current, and a second output for generating asecond current; and a output circuit, said output circuit producing saidoutput signal from said second current.